Smart Materials

Chapter 2

Smart Materials

2.1 Introduction

In this chapter, the analysis of rheological properties of a class of smart materials, is presented. These features will be adopted to introduce the “MRF-based haptic display” and the concept of “free hand haptic device”.

2.1.1 State-of-the-art

Smart materials, such as rheological

materials, exhibit a noticeable change of certain physical behaviour in response to an external stimuli. Rheological flu- ids, also termed controllable fluids, or rather Electro-Rheological Fluids (ERFs) and Magneto-Rheological Fluids (MRFs), are a particular class of smart materi- als, capable of changing their rheological behaviour when an external electric or magnetic field is applied [19, 20, 21]. The initial discovery of rheological fluids is credited to Willis Winslow who, in the 1940’s, described the first time the effects on Electrorheological fluids [84, 85]. Around the same time, in 1949, Jacob Rabinow

1The rheology, applicable to all materials from gases to solids, is the science that describes the interrelation between force, deformation and time and of matters.

described Magnetorheological fluids effects and developed the first MRF devices.

Although these controllable fluids have fascinated scientists, engineers, and inven- tors for nearly 60 years and they are present on the marked, their application field is restricted to several devices such as valves, brakes, clutches, dampers, in civil, mechanical engineering applications [49, 50, 73], [19, 63, 69, 76]. Rheological fluids are commonly used in the field of vibration control, in automotive area [27, 34] and aerospace industry [5, 23, 25, 39, 56]. The advantage of using controllable fluids is the possibility to design and realize a variety of real applications, by involving semi-active control, without additional mechanical parts [16, 36, 54]. On the other hand, the effects of rheological fluids can be combined with other actuator types such as electro-magnetic, pneumatic, or electrochemical actuators so that novel, hybrid actuators are produced with high-power density and low-energy require- ment [46, 51, 29, 37].

It is well known that conventional haptic devices are based on motor control to pro- vide force and torque. Clearly it’s not so easy for traditional device (e.g. PHANToM

° by SensAble, Delta c c ° by Force Dimension) to simulate accurate forces, viscosi- ties and behaviours of the virtual objects. For example, during multiple interactions with deformable objects, complex algorithms for controlling should be provided in order to simplify the VE [3, 58].

In this Thesis the possibility of using ERFs or MRFs in haptic interfaces, ex- ploiting their property of changing the rheological behaviour by tuning an external electric or magnetic field is explored.

In this scenario, the use of smart fluids as haptic displays can be an innovative and viable solution, because they allow mimicking different viscoelastic materials by tuning an external electric (for the ERFs) or magnetic (for MRFs) field.

Some authors [15, 18, 61, 78, 79] have already explored the possibility of using

rheological fluids in tactile displays, in particular, ERFs arranged or linked with

mechanical components. A prototype based on ERFs for blind people is a tactile

graphic I/O tablet [30]. Another example conceived for a medical teleoperation system is the MEMICA glove (acronym for MEchanical MIrroring using Controlled stiffness and Actuators), whose components are miniature electrically-controlled force and stiffness (ECFS) actuators that are based on the use of ERFs [4]. In addition a portable hand and wrist rehabilitation device based on MRFs was devel- oped [22].

2.1.2 Phenomenology of Rheological Fluids

Rheological fluids are generally non-colloidal

suspensions of micron-sized polariz- able suspended in a synthetic liquid and exhibit a rapid, reversible and tunable transition from a liquid to a near-solid state upon the application of an external field. More specifically, Electrorheological and Magnetorheological fluids are ma- terials that respond to an applied electric and/or magnetic field with a change in rheological behaviour. Typically, this change is manifested by the development of a yield stress that monotonically increases with applied field. Just as quickly, the fluid can be returned to its liquid state by the removal of the field, thereby being a re- versible phenomenon [59, 49, 83]. To better understand the functioning mechanism let us assume that this fluid is located in a gap between two plates (fig.2.1). We can conjecture ferrous electrodes or ferromagnetic plates capable to excite electrically or magnetically, respectively ERFs and MRFs. The fluid with its polarizable par- ticles is positioned in the air gap between both plates. The mobile surface moves horizontally being u is its velocity. The variable v, ranging from 0 and u, is the velocity of the fluid within the gap.

In the absence of an applied field a controllable fluid exhibits a Newtonian- like behaviour and flows freely being the polarizable particles randomly distributed throughput the fluid (2.1). A simple equation to describe Newtonian fluid behaviour

2A colloid is a finely divided, solid material, which when dispersed in a liquid medium, scatters a light beam and does not settle by gravity; such particles are usually less than 2 microns in diameter [41].

Figure 2.1: Simplified rheology formulation of controllable fluids.

is:

τ = µ dv

dy (2.1)

where τ is the shear stress exerted by the fluid, µ is the fluid viscosity, that is a constant of proportionality, and

is the velocity gradient perpendicular to the direction of shear.

Indeed, a Newtonian fluid, is a fluid that flows like water, being its shear stress linearly proportional to the velocity gradient in the direction perpendicular to the plane of shear.

If the fluid does not obey this relation, it is termed a non-Newtonian fluid, of which there are several types.

In the presence of an external (electric or magnetic) field the fluid does not obey

this relation and it develops a precisely controllable yield stress: the polarizable

particles in the gap align themselves in the same direction of the field, and created

particles chains restrict the movement of the fluid. The degree of change in terms of

yield stress is approximately proportional to the magnitude of the applied field. The

behaviour of rheological fluids is often represented as a Bingham plastic model [16,

47, 74, 76, 77]. In this model, having variable shear stress, the plastic viscosity is

defined as the slope of the measured shear stress versus shear strain rate data, and

Figure 2.2: Rheological behaviour of controllable fluids non excited (left - like New- tonian model) or excited (right - like Bingham plastic model).

the fluid is governed by Bingham’s equations:

τ = τ

(•) + µ dv

dy (2.2)

where τ

(or also tau

) is yield stress induced by magnetic (• = H) or electric field (• = E), is the viscosity and

(often indicated with γ) is the fluid share rate.

In fig.2.2 a representation of Newtonian and Bingham models expressed in term of shear stress rate function is shown.

Comparison between Rheological Fluids

ERFs are suspensions of electrically-polarizable particles, souch as semiconductor,

silica, barium or cornstarch, dispersed in electrically-insulating synthetic oil. Typ-

ical size of these particles is approximately 0.1 to 100 µm. Dually MRFs are syn-

thetic oil-based or water-based suspensions of magnetically polarizable particles

(size 0.05 − 1 µm). In this case these magnetic particles are smaller than those

found in typical ERFs. Unfortunately, the rheological behaviour and mechanism

used to activate both fluids are temperature dependent. The operating range is

approximately +10

to +90

C for non-ionic ERF (used in AC applications with

high frequency). Like ERFs, MRFs present a military operational temperature over

the range −40

to 150

C. In terms of their consistency or softness, controllable

fluids appear liquid in the off-state, exhibiting a comparable value of viscosity. In

Figure 2.3: Phenomenology of MRFs: Internal configuration in the absence of the magnetic field (on the left), in intermediate configuration (middle figure) and with a strong magnetic field applied (on the right).

this state both MRFs and ERFs reveal a viscosity ranging from 0.20 to 0.30 Pa · s with 25

C. Since the electric field is limited by the breakdown effect in ERFs, and by the magnetic saturation in MRFs, a maximal threshold of yield strength (latent viscosity) is achievable. When an electric and/or magnetic field is applied, the fluids turn from liquid to near solid in few milliseconds by changing significantly their apparent viscosity. When a maximum electric field is applied (approximately in 100 ms) ERFs exhibit a typical level of saturation yield strength of about 5 kPa, while MRFs, excited with saturation magnetic field, in near 10 ms show a yield strength up to 100 kPa.

In table 2.1 we summarize some differences between ERFs and MRFs [33].

2.1.3 ERFs and MRFs: Applicability to Haptic Interfaces

According to goal, an innovative application based on smart fluids for developing

haptic interfaces was investigated. The MRFs appeared more suitable for our appli-

cations than ERFs due its good yield stress range and response time. MRFs appear

to have a safer excitation field than ERFs in terms of direct interaction with the

specimen. An external magnetic field that energizes MRFs must be capable to pro-

duce a yield stress compatible with a rheological characteristic of some viscoelastic

materials, i.e biological tissues. The energization principle and a schematization of

the MRF is shown in fig.2.4.

Table 2.1: Comparison between ERFs and MRFs

ERFs MRFs

Dispersed Medium Polymers, Zeolites, etc. Iron, steel, ferrites

Particles size 0.1 − 100 µm 0.1 − 100 µm

Suspending fluid Oil, dielectric gel and other polymers

Synthetic oils, non polar and polar liquids,

water, etc.

Density (g/cc) 1 − 2 3 − 5

Off viscosity (mPa-s) at 25

C

50 − 2000 50 − 1000

Excitation field 3 − 5 kV/mm 150 − 350 kA/m

Yield stress induced τ

10 kPa 100 kPa

Typical excitation sources

High voltage Electromagnets and/or permanent magnets Typical Response

Time

100-200 ms 1-10 ms

Figure 2.4: Magnetic and rheological properties of MRF132LD.

By using the characteristics of the selected fluid, we can evaluate operating point of the MRF and consequently the magnetic (B) field able to determine the desired yield stress (τ

).

In our applications we used a commercial magnetorheological fluid marked MRF 132LD produced by Lord Corporation c °, Cary NC, USA.The main features of this fluid extrapolated from the magnetic and rheological characteristics (fig.2.4) and from the technical datasheet can be briefly summarized as follows:

Magnetic properties

saturation threshold: B

≈ 0.5 − 0.6 T;

relative initial permeability: µ

≈ 3.5;

maximum relative permeability: µ

≈ 7.4.

Mechanical and rheological properties:

response time t

≈ 10 ms ;

maximum yield stress τ

≈ 45 kPa.